This biography of one of the greatest minds in Western philosophy, from the author of Fermat's Last Theorem, brilliantly illuminates René Descartes through the few pages of a secret notebook that his fellow mathematician Gottfried Leibniz was able to briefly observe decades after Descartes' death in 1650. Leibniz immediately recognized some of Descartes' coded annotations as symbols of the Rosicrucians—Leibniz was also a member of this secret order—and was able to decipher tantalizing clues to the true depth of Descartes' insights.